As digital data services proliferate, the need for data channels to carry these services into homes and businesses likewise increases. It has become common to install special wideband transmission facilities in those places where such wideband digital services are desired. These special transmission facilities are expensive, require continuous maintenance, often in the outside plant portion of the facility, and require expensive terminal equipment. It would be of considerable economic benefit if the twisted-pair telephone wires currently extending to virtually all of the homes and businesses in the country were able to carry such wideband digital services.
It has long been known that noisy transmission channels of restricted bandwidth can be used to carry wideband digital signals with reasonable fidelity by the use of channel equalizers, i.e., circuits which compensate for the signal deterioration which takes place during transmission. This signal distortion can be represented by the impulse response of the transmission channel. The sampled impulse response includes a positive maxima which is the preferred signal sample and is called the cursor sample. The impulse response can thus be divided into a first region preceding the cursor sample, into which all pre-cursor samples fall, and a second region following the cursor sample, into which all post-cursor samples fall. Pre-cursor samples can be compensated for by means of an in-line filter in the received signal path. Post-cursor samples, however, are not so readily compensated for, particularly if such post-cursor samples are prolonged over a very large number of pulse periods.
Digital equalizers for compensating for post-cursor samples have often taken the form of adaptive digital decision feedback equalizers (DFEs) using a finite impulse response (FIR) filter in the feedback path. Such DFEs sample the received signal at regular pulse intervals, delay each sample, operate on each delayed sample to produce a compensation sample, and subtract the compensation sample from the incoming post-cursor pulse sample to substantially remove such post-cursor samples. For wideband digital signals, the impulse response of the channel extends over very many pulse intervals, requiring post-cursor compensation for that number of pulse intervals. This is typically accomplished by providing a plurality of compensating samples, using a tapped delay line. For very wideband signals, the number of taps on the delay line becomes so great that construction of the equalizer may not be economically feasible. For a 800 Kbs digital signal transmitted over a conventional twisted telephone pair, for example, the impulse response could very well extend over more than a hundred pulse intervals. In order to compensate for the distortions in such a channel, the FIR filter in the feedback path would have an exorbitantly high number of delay line taps and hence might not be economically feasible for many applications.